Superior adsorption performance of citrate modified graphene oxide as nano material for removal organic and inorganic pollutants from aqueous solution

This work addressed one step preparation method to form a novel nano material composite of graphene oxide nanosheet (GO) functionalized with low-cost tri-sodium citrate (C), using, teteraethylorthosilicate (TEOS) as a cross-linker. The prepared composite (GO–C) was characterized using various advanced techniques. Among these techniques, the TGA provided interesting information concerning the functionalization process. Within this process, the (–OH) groups that located at the GO-surface were consumed in the modification process which leads to increase the thermal stability of the resulted composite. Cationic organic methylene blue (MB) and crystal violet (CV), and inorganic copper (Cu2+) and cobalt (Co2+) pollutants were displayed as a model to assess their removal performance by the developed composite (GO–C) from aqueous solution, through batch technique. According to Langmuir isotherm the GO–C present an excellent adsorption capacity for MB (222.22 mg g−1), CV (270.27 mg g−1), Cu2+ (163.4 mg g−1) and Co2+ (145.35 mg g−1) which were more than the adsorption capacities found in literature. Additionally, the regenerated composite presents higher removal ability than the original composite.


Experiment
Preparation of GO and GO-C composite. GO was prepared as investigated by our previous work 28 .
The GO-C was simply prepared through one-put mixing process. Briefly, tri-sodium citrate (10 g) was added to 800 ml distill water and stirred variously until complete dissolution. To this solution, 150 mg GO were added and the stirring continued to form homogeneous suspension of the GO (solution a). In another beaker, 5.0 ml TMOS were added to 50 ml ethanol (solution b). Consequently, (solution b) added dropwise to the (solution a), the temperature of the final solution raised up to 60 °C and left on the stirrer for 24 h. The solid was filtrated and washed several times with distilled water and stored for further use. Figure 1, present a schematic illustration of GO-C composite preparation, adsorption and regeneration/reused steps.   www.nature.com/scientificreports/ Raman spectra. Raman spectroscopy is exceedingly employed to explore the structure of the carboneous material. It is observed that, with increase the disorderly of the graphite, the bands at 1352 cm −1 (D-band) and 1598 cm −1 (G-band) 28 become more-broader. Additionally, I D /I G , was employed as the sp 2 /sp 3 ratio of the hybrid carbon 32 . Moreover, I D /I G also express as the defects caused by the oxygen functional groups that linked to carbon skeleton and more, indicative of the integrity of the aromatic structure. The Raman spectra of the analyzed samples GO, GO-C, GO-C-MB and GO-C-CV, are demonstrated in Fig. S3. The locations of D and G-band, I D /I G ratios and FWHMs are summarized in Table S1. As been noted from the Fig. S3, the I D /I G ratio of the GO decrease after the modification process which indicate that the functionalization process may be repair of some defects located in the GO surface. After adsorption process the I D /I G ratio tend to increase again which indicate consumption of the modifier in the adsorption of the dye.
TGA analysis. The thermo-gravimetric analysis is a useful technique to follow up the loss of the sample weight with further increase in the temperature. The thermal decomposition of GO, GO-C composite and GO-C-MB complex was carried out, as shown in Fig. 3b. And the decomposition temperature range, %wt loss and related degradable species was listed in Table S2 Fig. 3b.
On the other hand, the thermograph of the GO-C present six decomposition stages, as explored in Fig. 3b. As pointed from the thermogram of the GO-C, Fig. 3b, the functionalization process leading to reduce the amount of the bounded surface water (Table S2). This is may be attributed to, most the hydroxyl (-OH) localized on the GO surface (which were responsible on the interaction with surrounding moisture via H-bonding) were contributed in the modification process as suggested (see Fig. 1). This can be noted from the thermograph (Fig. 3b) and the Table S2, where the GO loss 19.6% from its weight in first degradation stage, 28-89 °C, which crossponding to liberation of adsorbed surface water molecules. Followed by the main decomposition step (89-158 °C) which related to the pyrolysis of GO surface (OH) groups (weight loss = 20.50%). Whereas, in case of the GO-C, the amount of liberated moisture was evaluated (8.92%, 27-92 °C) followed by the second pyrolysis step which recorded a very low weight loss (1.78%, 92-139 °C), see (Fig. 3b) and Table S2. This may be clarifying that, the OH groups decorated the GO surface were shared in the modification process. Moreover, the GO-C provided high thermal stability than the GO, where, the GO-C possessed the main degradation stage in rang (324-474 °C) with 14.05% weight loss. This obtained result good agreement with literature 23 .
Further, by mixing of the GO-C with the dye solution, the dye species will be adsorbed on the different function groups (-OH, -COOH and -COONa) to form (GO-C-MB complex). This behavior may be resulted in covering GO-C with the dye species and isolated the composite far from the temperature effect leading to the increase of the thermal stability of the GO-C-MB, refer Fig. 3b and Table S2. Therefore, the GO-C-MB show the main thermal decomposition step in range (470-668 °C) with weight loss ratio 20.70%.
Energy-dispersive X-ray spectroscopy (ESD) analysis. The importance of the EDS analysis is demonstrating on the elemental composition of the fabricated material. Graphene oxide is a carbonaceous material mainly composed of C and O-atoms. Herein, we modified the GO with tri-sodium citrate and using tetraethylorthosilicate as a binder. Therefore, the elemental analysis of GO-C shows the presence of Na and Si-atoms in the resulted EDS analysis, see Fig. 4. By mixing the adsorbent with the adsorbate solution the %At of C, O, Na and Si will be altering and more S-atom appears in the new EDS which confirm the adsorption of the dyes over the adsorbate. Moreover, the appearance of Cu and Co in the elemental analysis of the composite after contacting with their aqueous solutions by using Oxford energy-dispersive X-ray (EDX) spectrometer provide their capturing with the GO-C composite, Fig. 4.
Adsorption performance of the GO-C composite. In order to evaluate the adsorption efficiency of the GO-C and according to the nature of modified active groups used, four cationic species were select two of them related to organic dyes (MB & CV) and the other inorganic species (Cu 2+ & Co 2+ ). This is due to cover the most main pollutants that results from the industrial activities. www.nature.com/scientificreports/ that it affected on the applicable ability of the prepared materials. Batch adsorptions were studied by stirring the aqueous dye (MB or CV) or heavy metal (Cu 2+ or Co 2+ ) solution with adsorbent for time interval (0.16-30 min). The relation between the contact time and the removal percent was explored in (Fig. 5a). The results showed that, the adsorption kinetics was highly fast even at early adsorption stages. This behavior may be attributed to the high affinity of active function groups (-COO − Na + ) of the citrate towards the cationic pollutant species.
Adsorption kinetics. The studied of adsorption rate is required to design a suitable adsorption system, therefore, pseudo first order and pseudo second order were utilized. The adsorption of the pollutants over the GO-C show rapid equilibrium, therefore, the Pseudo first order kinetic model will not be involved. The linear relationship of the used pseudo second order model (see Table S3) presented in (Fig. 5b), and the experimental and calculated data obtained were listed in Table S4. The adsorption rate of the pollutant molecules (MB, CV, Cu 2+ , Co 2+ ) over the GO-C show relative stability over all the investigated time, therefore, the pseudo first order was not investigated. On the other hand, at using the pseudo second order the relation coefficient (R 2 ) of the hazardous were > 0.99. Moreover, the calculated adsorption capacity related to the adsorbed molecules were very www.nature.com/scientificreports/ closer to their values obtained from the experiments. This proves that, the pseudo second order best fitted the experimental data.
Effect of initial concentration on the removal efficiency. The impact of initial concentration on the removal performance of the different cationic species, MB, CV, Cu 2+ , Co 2+ by GO-C were investigated in the range (10-50 mg L −1 ) for MB and CV and (50-150 mg L −1 ) for Cu 2+ and Co 2+ , as explored in Fig. 6a. It was seen that the elimination efficiency of the two dyes decrease significantly 88-49% (MB) and 86-60% (CV)as the initial dye concentration increase in rang (10-50 mg L −1 ) for both dyes, 92.54-49.59% (Cu 2+ ) and 85.21-44.12% (Co 2+ ) as the initial heavy metal ion concentration increase in range (50-150 mg L −1 ) for both metal ions. This is referred to, at low pollutants concentration, the number of the adsorbent active sites were sufficient for adsorb the pollutant species. As the initial adsorbate concentration increase the number of available active site will be decrease compared to the number of adsorbate species.
Adsorption isotherms. Adsorption isotherms are required to ascribe the relation among the quantity of the adsorbed species and its concentration in the aqueous solution at reaching equilibrium at constant temperature. The parameters calculated from various models supply interested information about the adsorption mechanism, the surface character and affinity of the adsorbent. The most used models named as Langmuir and Freundlich models, see Table S5.
The graphs of C e versus C e /q e and log C e versus log q e , Fig. 6b,c, respectively. The evaluated parameters of the models and correlation coefficient (R 2 ) for the alter adsorptions species are recorded in Table 2. According to R 2 values, the Langmuir equation shows a better fit than the Freundlich with superior adsorption performance for MB (222.22 mg g −1 ), CV (270.27 mg g −1 ), Cu 2+ (163.40 mg g −1 ) and Co 2+ (145.35 mg g −1 ) Table 2. Consequently, the significant character of the Langmuir isotherm can be expressed in terms of dimensionless separation parameter, R L , Fig. 6d, which is term of the isotherm shape that indicates if the adsorption system is favorable or unfavorable. R L is defined as (Eq. 1): where, b is the Langmuir constant. In this work R L values located among 0 and 1 (0.12 for MB, 0.11 for CV, 0.09 for Cu 2+ and 0.12 for Co 2+ ) as indicated in Table 2 which demonstrated applicable adsorption process. Moreover, Freundlich model is property by 1 n heterogeneity factor; where, the sites on the surface were not have the same binding energy. The values of 1/n between 0.1 < 1 n < 1.0 that explored suitable adsorption of the pollutants onto the adsorbent.    Fig. S4a. The obtained data showed that the increase in the quantity of the adsorbent dose (4-8 mg) (for the dyes) and (2.4-6 mg) for metal ions was followed with significant increase in the removal percent for all species. Further increase in adsorbent dose amount presented equilibrium adsorption rate. This may be ascribed as; at low adsorbent dose the GO-C layers were completely separated and the active sites fully exposed to the contaminated solution which enhance the adsorption efficiency see SEM images. On the other hand, the increase in the adsorbent dose will cause GO-C agglomeration which will lead to obscure most of the active sites available for adsorption of the adsorbed species (Fig. S4b,c).
Effect of aqueous solution pH on the removal efficiency. The pH of the adsorption solution is considered an interested condition in the uptake process. Where, it can regular the mechanism for elimination of pollutants www.nature.com/scientificreports/ since the pH value can impacted on the active sites located on the adsorbate surface. The influence of the solution pH on the uptake percent of the GO-C was plotted in (Fig. 7a). It was clearly seen that, the adsorption percent increase sharply ((24-94% (MB), 22-91% (CV), 7-94% (Cu 2+ ) and 16-87% (Co 2+ )) with further increase in the pH value in the range 1.6-11.5 (MB), 2.15-9.95 (CV), 1-6 (Cu 2+ ) and 2-8 (Co 2+ ). On focus, the point of zero charge (PH ZPC ) is known as the pH value at which the net charge of the adsorbent, due to H + and OH − ions interaction with functional groups located on its surface, is null. Hence, we can determine of PH ZPC by plotting ΔpH (pH f -pH i ) against pH i , where pH i , the initial pH value of the solution and pH f , the pH of the solution after the treatment process. The data obtained from the experiments were plotted in Fig. 7b. The results obtained showed that the composite (GO-C) is completely ionized even at very low pH (1.6). This regime is attributed to the GO was modified with tri-sodium citrate which has three sodium carboxylate (-COO − Na + ) groups, these groups are already ionized. Therefore, the presentation of the GO-C in the dye solution will increase the negativity of the adsorption solution.
Based on the pervious information, the mechanism of adsorption of the cationic species (MB, CV, Cu 2+ and Cu 2+ ) could be suggested. At low pH value H + -ion compete the dye species for the available active sites (-COO − Na + ) on the adsorbent and form (-COOH) and so the affinity of the adsorbent towards the cationic species decreases. With increase in the solution pH value the concentration of H + -ion decrease and the competition for the active sites will be minimized and so the adsorption percent of the cationic species enhanced. Moreover, the further increase in the pH over 10 (MB) and 8.5 (CV) the removal percent of the dye tend to be stable. This may be attributed to at high alkaline solution the activated function groups (-COO − Na + ) introduce over the adsorbent suffer from low ionization by the common ion effect due to concentration of Na + -ion of the solution 33 . Moreover, based on the EDS analysis, Fig. 4, of the GO-C showed the presence of Na + which related   www.nature.com/scientificreports/ to the (COO − Na + ) groups of the citrate molecules. After mixing the GO-C composite with the aqueous solutions of MB, CV, Cu 2+ and Co 2+ , the %At of the Na + will highly reduced or almost disappear which indicated that the pollutant species adsorbed via cationic exchange mechanism.
The effect of the aqueous solution temperature on the removal percent. To evaluate the impact of the temperature on the adsorption behavior of the adsorbent, the adsorption processes were carried out at different temperature values ranged from 30 to 95 °C for the dye and 25-60 °C for the heavy metal ions. The relation between the temperature and the adsorption performance (% R) were plotted in Fig. S5a. The data obtained revealed that, there was little variations in the removal percent of the two dyes as the temperature differ from 30 to 95 °C. On the other hand, there is a slow increase in the up-take percent of the metal ions with further increase in solution temperature from 25 to 60 °C.
Thermodynamic studies. The standard thermodynamic parameters of the sorption process represented through Gibbs free energy, ΔG°, Enthalpy, ΔH°, and Entropy, ΔS°. These represent the main thermodynamic function to assess sorption reaction. Vant Hoff equation was employed to evaluate Gibbs free energy as in Table S6. The enthalpy change, ΔH° and the entropy, ΔS° were obtained from the equation presented in Table S6. The plot of ln K d versus 1/T, a straight line was obtained as illustrated in Fig. S5b. From the slope, the value of ΔH° was calculated at 25 °C as shown in Table S7. While Entropy change, ΔS° is obtained from the intercept as shown in Table S7. It is clear that the sorption of Cu and Co ions is endothermic and spontaneous reaction. Further, the positive entropy changes indicate the randomness of the adsorption process.
Regeneration and reusability. Five adsorption-desorption runs were carried out for test the reusability performance of the adsorbent, as indicated in Fig. 8. The results indicate that, there was an increase in the adsorption percent of the all pollutants upon regeneration and reuse over five cycles. This may be attributed to the using of the NaOH will be activated extra function groups located on the GO-surface, refer Fig. 1. Hence, the number of available active site increase, consequently, the affinity towards the cationic pollutants will be performed, thereafter, the adsorption percent will be enhanced. Moreover, the regeneration-reusability function of GO-C, will reduce the applicable cost.  www.nature.com/scientificreports/ monitored visually and with the UV-Vis spectroscopy, as present in Fig. 9a-c. The results showed that, the GO-C composite can completely remove the MB and CV dyes (binary system) from the aqueous solution, and this result confirmed from the completely diminished of the MB and CV peaks at the end of the adsorption process, see Fig. 9a. Moreover, upon mixing of cationic dyes (MB or (MB&CV)) with the anionic dye (MOdye), they yielded solution with green color, as indicated in Fig. 9b,c. By treating the previous solutions with the GO-C composite the color of the blended solution turned into orang, crossponding to the color of MO-dye, and more, the UV-Vis spectra indicated the completely disappearance of the peaks related to the MB and CV, refer Fig. 9b,c, after the treatment process. These results provided that the GO-C composite can adsorb cationic dyes from binary system, and select adsorption of singlet and binary cationic dyes mixed with anionic dye. Moreover, the GO-C was applied for cleaning real wastewater sample as shown in Fig. 9d as demonstrated from the Fig. 8d, the GO-C composite was success in purification of the treated real sample. These results indicates that the asprepared GO-C nanocomposite could be applied for treatment of the industrial effluents.   Table 3. Therefore, we can conclude that the GO-C can be consider as a superior adsorbent due to its easy, low-cost synthesis process and excellent affinity for wide range pollutants.

Conclusion
Citrate modified graphene oxide (GO-C) was simply prepared and investigated to eliminate dyes and heavy metals from aqueous solution. The structure of the composite was characterized employing SEM, OM, FTIR, Raman, EDS. According to the TGA analysis, the -OH groups located on the GO basal plane share in the modification step. This manner highly affected on the thermal properties of the GO-C composite compared with the GO. Moreover, the adsorption of the dye by the GO-C was found also enhance the thermal stability of the GO-C-MB complex. Attributed to the modification of the GO with sodium citrate, the GO-C showed a high rapid kinetics and an excellent affinity for MB, CV, Cu 2+ and Co 2+ , the crossponding adsorption capacity performance according to Langmuir were 222.22, 270.27, 163.4 and 145.35 mg g −1 under a single system which were larger than presented in the literature. The composite shows additional properties, that the it can selective adsorption of the cationic dyes in singlet and binary system from aqueous solution. Additionally, the GO-C regenerated/reused over five cycle times with increasing in the adsorption percent than related to the pristine composite.

Data availability
All data generated or analyzed during this study are included in this published article and its supplementary information files.